Some novels are obviously unfilmable: To the Lighthouse and Gravity’s Rainbow, for two. But can a novel be unnarratable? Until recently, I might have nominated the four delightful murder mysteries written by Sarah Caudwell—three published in the 1980s and the fourth just before her death at age 60 in 2000—as unadaptable to the audiobook format. It’s not that they have some untranslatable visual element or experimental structure. Caudwell’s crime novels are frothy, witty confections that follow the familiar conventions of classic British mysteries, spiced with lashings of Wodehouse-style comedy.

However, what makes these mysteries a challenge to any audiobook performer is their narrator, Hillary Tamar, an Oxford don specializing in medieval law. Throughout the four novels, Caudwell never specifies Hillary’s age or gender—an omission so skillfully executed that many of the original readers of the mysteries never even noticed. In an obituary for Caudwell, a bookseller recalled overhearing fans discussing her books in the shop: “One reader would say ‘he,’ another ‘she,’ and they’d stare at each other. It was a delicious joke.” Catching on to the joke as you read the novels, beginning with 1981’s Thus Was Adonis Murdered, is one of the pleasures of Caudwell’s work.

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A preface to the just-released audio recordings of the first two Hillary Tamar books, Adonis and The Shortest Way to Hades—recordings of the final two books, The Sirens Sang of Murder and The Sybil in Her Grave, will be released in December—tips off listeners to Caudwell’s narratorial trickery. Narrator Lorna Bennett explains that “keeping Hillary’s gender ambiguous in these audiobook editions proved virtually impossible.” Bennett doesn’t give herself enough credit, and the prefaces seem unnecessary. Her husky, stuffy, precise portrayal of Hillary seems perfectly suspended between male and female or, as Caudwell herself put it, firmly located in an identity that supersedes both: the sort of Oxford don “who starts being elderly at the age of 22.” It’s quite easy to imagine two separate listeners to these audiobooks coming away with different notions about Hillary’s gender.

The novels involve Hillary’s friendship with five junior barristers working in London’s venerable legal district. They specialize in some of the most tedious aspects of the law (taxes and trusts), which nevertheless provide abundant occasions for skullduggery, jokes, and homicide. In the Venice-set Thus Was Adonis Murdered, one of the friends, the hapless and pragmatically challenged Julia Larwood, is suspected of murder because her copy of the Finance Act is found in the victim’s bedroom. A plot point turns on the law of domicile and how it can be used to “mitigate one’s prospective liability to capital transfer tax.” Also of interest is a tax dodge nicknamed “the impecunious husband,” which the young barristers discuss with absolute seriousness.

All this was familiar ground to Caudwell, who studied at Oxford, joined the Chancery Bar in 1966, and worked for Lloyds bank. Her real name was Sarah Cockburn, and she was the product of a remarkable lineage. She shared a father—the leftist journalist Claud Cockburn—with the celebrated journalists Alexander, Andrew, and Patrick Cockburn. The actor and director Olivia Wilde is her niece. Caudwell’s mother, Jean Ross, was also a journalist who covered the Spanish Civil War for the Daily Worker and remained a committed Communist until her death in 1973. But Ross, who also sang in nightclubs in Weimar Germany, is probably best known as the inspiration for Sally Bowles, the hedonistic fictional character created by Christopher Isherwood and immortalized in the musical Cabaret.

A staunch but mischievous feminist, Caudwell conspired in pranks that helped to open the all-male Oxford Union, a debating club, to female speakers. She was famous for smoking a pipe, which she would often light up in restaurants, then stuff, still lit, into her handbag after being informed that no smoking was allowed. Somehow the handbags never caught fire, but they often smoldered through the meal.

One of the most refreshing qualities of Caudwell’s mysteries is their blithe, unlabored integration of old-fashioned, Agatha Christie–style puzzles with more contemporary mores and values. Her characters take same-sex relationships and bisexuality as a given. Julia, for all her ineptitude, pursues a series of successful seductions (mostly, but not entirely, of beautiful young men) without expressing the slightest interest in marrying any of them. Caudwell doesn’t preach a feminism in which one’s gender need not determine one’s career, sex life, or skill at solving mysteries. She simply shows her readers a world in which all of that is already true.

The books are also extremely funny in a particular and very British vein. Hillary professes to be baffled by the exotic expressions and customs of one of the barristers because the young man went to Cambridge instead of Oxford. Characters drop such Wildean epigrams as “I am always pleased to see her, for we have several enemies in common.” Bennett deftly handles all of this, from the plummy upper-class accents of the clients to the incessant malapropisms of the barrister with the lamentably inadequate Cambridge education.

Best of all, her performance of Hillary sacrifices none of the character’s enigmas, leaving the series’ most intriguing mystery of all safely unsolved. It’s a rare piece of fizzy entertainment that succeeds so ingeniously in capsizing common assumptions about gender and sex—and Caudwell wrote four such books. Their arrival on audiobook, at long last, is even more welcome than a scheme to dodge capital transfer tax.